The present invention relates to line scan image sensors and, more particularly, to a solid state line scan image sensor having a variable resolution.
Line scan image sensors such as solid state photosensors using charge coupled device (CCD) transfer are used to scan an image and produce data in the form of successive charge packets representing a linear portion of the image. Such sensors are frequently used to scan successive lines of an image in scanning applications such as facsimile or optical character recognition.
A typical CCD line scan image sensor has a row of photoelements for sensing the light intensity of a predetermined number of samples or picture elements (pixels) of an image. The number of pixels per unit length on the page is a measure of the resolution of the device and determines the number of photoelements necessary to scan the full width of the page to be imaged. A device having 200 pixel per inch resolution, for example, requires 1700 photoelements to scan one line of an 81/2 inch wide page, while a device having 400 pixel per inch resolution requires twice as many photoelements to scan the same page width. Similarly, the resolution of the line scan image sensor determines the amount of data (number of charge packets) per line of the image and the time required to scan a line for a fixed data rate.
Systems requiring a high degree of resolution tend to be slow, whereas high speed readers often lack the degree of resolution required in certain applications. Accordingly, it is desirable to provide a sensor which is operable at several discrete spatial sampling frequencies in order to permit a selectable transmission rate. The typical prior art approach is to group the charge packets produced by the individual photoelements by combining them either on the image sensor chip or in an off-chip resampling circuit. Thus, for example, a 2:1 reduction in resolution may be achieved by grouping pairs of adjacent charge packets to decrease the data transmission time by one half for a fixed sample rate.
Charge packet grouping or combining in a CCD has been used successfully to achieve 2:1 and 3:1 reductions in resolution and spatial sampling frequency. However, one of the reductions of interest, a 4:3 reduction, has not been achieved using conventional grouping techniques. To my knowledge, attempts at achieving this desired reduction ratio using off-chip resampling techniques have also not been successful thus far.
Theoretically, it is possible to provide a line scan image sensor having a resolution of 1200 pixels per inch, and to operate it in a 400 pixel per inch mode by combining charge packets into groups of three, or to operate it in a 300 pixel per inch mode by combining charge packets into groups of four. This scheme, however, requires a very high resolution line scan sensor which would be both expensive and complex.